For microwave transistors, whether discrete or in integrated circuits, the gate length measured in the direction source-drain is critical, in the submicronic field. Optical masking equipment is the most common, and is suitable for mass production, but it cannot mask gates of smaller length than about 1 micron. To construct so-called submicronic transistors, electronic means of masking must be used, which are much more expensive and above all much slower, but enable gates 0.2 micron in length to be constructed.